This invention relates to ballast circuits, and more particularly, to improved D.C. resistive ballast circuits for high pressure sodium vapor lamps.
High pressure sodium vapor lamps have found widespread use during the past decade for commercial lighting applications, especially outdoor lighting. Such lamps are described in U.S. Pat. No. 3,248,590 of Schmidt, entitled "High Pressure Sodium Vapor Lamps", and which is assigned to the same assignee as the present invention.
High pressure sodium (H.P.S.) vapor lamps typically utilize an inner ceramic arc tube having an appropriate metallic fill as its light source. A ballast circuit is provided to supply the necessary voltage to the H.P.S. lamp to cause its initial starting and operating arc conditions. U.S. Pat. No. 4,223,247 discloses a ballast circuit adapted to receive an A.C. power source applied across its input terminals and generate the necessary starting and operating voltages for the metal vapor discharge lamp.
The ballast circuit of U.S. Pat. No. 4,223,247 provides an A.C. operational voltage to the arc tube and embodies a transformer that is connected to one side of the path of the applied A.C. power. The current flow through the transformer creates resistive heating within the transformer which contributes to the reduction of the operational life of the transformer. It is desired that means be provided which reduce the resistive heating of the transformer so as to increase the expected operational life of the transformer.
The A.C. operating voltage applied to the arc tube causes two electrodes, internally connected within the arc tube, to alternately serve as the anode and the cathode of the arc tube. With regard to certain parameters of the arc tube, it is desired that one electrode serve only as the cathode while the other electrode serve only as the anode both in the arc tube. To accomplish such a fixed cathode and anode orientation, it is desired that a ballast circuit provide a D.C. operating voltage to the arc tube.
With further regard to the ballast circuit for the high pressure sodium lamp, the inductive transformer while assisting in providing a desired starting voltage to the arc tube has disadvantages with regard to its physical size and its previously discussed inherent resistive heating. It is desired that ballast means, other than inductive, be provided to assist in the generation of the starting voltage for the arc tube.
Still further, the power dissipation of the ballast circuit exclusive of the ballast resistor is considered important in that if it can be reduced, the temperature of the remainder of the circuit can be reduced, which is beneficial to the lifetime of some components. It is considered desirable that means be provided which reduce the power dissipation of the ballast circuit exclusive of the ballast resistor and that such means be provided to position the ballast resistor apart from the remainder of the circuitry so that its dissipation will not cause objectionable heating of the remainder of the circuitry.
Accordingly, objects of the present invention are to provide various ballast circuits for high pressure sodium (HPS) lamps which: (1) reduce the resistive heating of the transformer which assists in generating the high voltage starting pulse for the HPS lamp; (2) develop, along with the reduced resistive heating of the transformer, a D.C. operating voltage which is applied to the H.P.S. lamp; (3) develop a starting voltage which is applied to the HPS lamp without the need of a transformer; and (4) reduce the power dissipation of the ballast circuit itself exclusive of the ballast resistor.
These and other objects of the present invention will become apparent upon consideration of the following description of the invention.